Make ecdsa.saw proof work again.
Make ecdsa.saw proof work again.

This adapts the proof script to GaloisInc/saw-core#90, which
removed the `bitvector` type synonym from saw-core.

GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script,GaloisInc/saw-script